1. Field of the Invention
The present invention generally relates to a level adjustment method and a wavelength division multiplexing device and system. More particularly, the present invention is concerned with a level adjustment method for adjusting the level of an input signal, and a device and a system using such a method.
Recently, as a rapid increase in a demand for communications due to advance of intelligent societies, many communication channels have been newly installed. However, installation of optical fiber cables costs a great deal. Thus, it is requirement to efficiently utilize the existing optical fiber cables. Wavelength division multiplexing (WDM) is the main current in the development of efficient utilization of the existing cables and increase in the aggregate number of channels per fiber.
2. The description of the Related Art
FIG. 1 is a block diagram of a communication system using WDM transmission devices. The communication system shown in FIG. 1 includes WDM transmission devices 1a and 1b. Transmission devices 2a-1 through 2a-3 are connected to the WDM transmission device 1a, and transmission devices 2b-1 through 2b-3 are connected to the WDM transmission device 1b. A plurality of terminals 3a are connected to the transmission devices 2a-1 through 2a-3, and a plurality of terminals 3b are connected to the transmission devices 2b-1 through 2b-3. Each of the transmission devices 2a-1 through 2a-3 multiplexes signals from the terminals 3a by time-division multiplexing. Similarly, each of the transmission devices 2b-1 through 2b-3 multiplexes signals from the terminals 3b by time-division multiplexing.
The transmission devices 2a-1 through 2a-3 send multiplexed optical signals respectively having different wavelengths λ1, λ2 and λ3 to the WDM transmission device 1a. Similarly, the transmission devices 2b-1 through 2b-3 send multiplexed optical signals respectively having different wavelengths λ1, λ2 and λ3 to the WDM transmission device 1b. A repeater device 4 is provided between the WDM transmission devices 1a and 1b. The repeater device 4 may be omitted when the WDM transmission devices 1a and 1b are close to each other so that there is no need to repeat the optical signals transmitted therebetween.
The WDM transmission device 1a multiplexes the optical signals of the wavelengths λ1, λ2 and λ3, and sends the multiplexed optical signal thus obtained to the opposing the WDM transmission device 1b via the repeater 4. The WDM transmission device 1b demultiplexes the received optical signal into optical signals of the wavelengths λ1, λ2 and λ3, which are then supplied to the transmission devices 2b-1, 2b-2 and 2b-3, respectively. The transmission devices 2b-1 through 2b-3 separate the received signals by time-division multiplexing, the individual signals thus obtained being supplied to the terminals 3b. 
FIG. 2 is a block diagram of a WDM transmission device 10, which corresponds to the WDM transmission device 1a or 1b shown in FIG. 1. The WDM transmission device 10 is made up of a transmission unit and a reception unit. The optical signals of the wavelengths λ1, λ2 and λ3 are applied to the transmission unit of the WDM transmission device 10 via optical variable attenuators 20-1 through 20-3 provided outside of the device 10. The transmission unit includes a wavelength multiplexer 12 and an amplifier 14 for transmission. The wavelength multiplexer 12 multiplexes the optical signals of the wavelengths λ1, λ2 and λ3 respectively coming from the attenuators 20-1 through 20-3, and outputs the multiplexed optical signal to the amplifier 14. Then, the amplifier 14 amplifies the optical signal, and outputs the amplified optical signal to an optical transmission path (optical fiber). The wavelength multiplexer 12 includes, for example, a WDM coupler utilizing a grating or the like.
An optical signal transmitted over the wavelength-multiplexed optical signal is applied to a reception unit of the WDM transmission device 10. The reception unit includes an amplifier 18 for reception, and a wavelength demultiplexer 16. The amplifier 18 amplifies the received optical signal, the amplified signal being applied to the wavelength demultiplexer 16. Then, the wavelength demultiplexer 16 demultiplexes the received optical signal into optical signals of the wavelengths λ1, λ2 and λ3, which are then supplied to, for example, the transmission devices 2a-1 through 2a-3 shown in FIG. 1.
A factor used to evaluate the quality of transmission in the system including the WDM transmission device 10 is an optical signal-to-noise ratio (OSNR). It is desired that the OSNR is high and uniform to the optical signals of the wavelengths λ1, λ2 and λ3 on the receive side.
FIG. 3 shows spectra of lights that are input and output signals of the WDM transmission device 10. In FIG. 3, only four wavelengths are illustrated for the sake of simplicity. However, the number of wavelengths is not limited to four.
Part (A) of FIG. 3 shows a spectrum of light that is the output signal of the amplifier 14 for transmission shown in FIG. 2. The four peaks of the wavelengths λ1-λ4 have been subjected to a level adjustment, and have almost equal peak levels. The optical signals exhibiting the spectrum shown in part (A) of FIG. 3 are transmitted over the optical fiber. In this case, the output signal of the amplifier 18 for reception shown in FIG. 2 has a spectrum of light as shown in part (B) of FIG. 3. A gentle envelope having a mountain shape is a spectrum of an amplified spontaneous emission light (ASE light) accumulated in the optical signal due to amplifiers on the transmission path. Four sharp peaks of light shown in part (B) of FIG. 3 have the wavelengths λ1-λ4. It is to be noted that the four peaks shown in part (B) have different levels. It will be noted that an ASE light introduced in the amplifier 14 is neglected in the illustration of part (A) of FIG. 3.
The OSNR corresponds to the difference between the peak level of the optical spectrum and the level of the ASE light. For example, as shown in part (B) of FIG. 3, the OSNR of the optical spectrum of the wavelength λ4 is indicated by an arrow 22.
Thus, as shown in part (A) of FIG. 3, even if the amounts of attenuation by the optical variable attenuators 20-1 through 20-3 are varied so that the lights of the wavelengths λ1-λ4 have a constant level, the OSNRs of the wavelengths λ1-λ4 are not constant. This is due to the following. First, the ASE light is accumulated in the light each time the light passes through the amplifier or the like. Second, the gain of each amplifier has a wavelength dependence. Third, the optical fiber has a loss of wavelength dependence.
The levels of the optical variable attenuators 20-1 through 20-3 shown in FIG. 2 are adjusted taking into consideration the above factors. This adjustment on the transmission side is called pre-emphasis control.
FIG. 4 is a block diagram of a WDM transmission device 30 having a different configuration as that of the WDM transmission device 10. The WDM transmission device 30 includes, in the transmission unit, variable attenuators 36-1 through 36-3, the wavelength multiplexer 12, the amplifier 14 for transmission, an optical coupler 32, and a spectrum monitor unit 34. The reception unit of the WDM transmission device 30 is the same as that of the device 10 shown in FIG. 2.
Lights of the wavelengths λ1, λ2 and λ3 pass through the variable attenuators 36-1 through 36-3 of the WDM transmission device 10, and are applied to the wavelength multiplexer 12. Then, the multiplexer 12 multiplexes the wavelengths λ1, λ2 and λ3 of the optical signals, and outputs the multiplexed optical signal to the transmission amplifier 14. Then, the amplifier 14 amplifies the received signal, the amplified optical signal being output to the optical fiber via the coupler 32.
The coupler 32 outputs a part of the optical signal from the amplifier 14 to the spectrum monitor unit 34, which may be formed by a spectrum analyzer. The spectrum monitor unit 34 defines target levels with regard to the optical signals of the wavelengths λ1, λ2 and λ3. The spectrum monitor unit 34 measures the wavelengths, levels and OSNR of the light components contacted in the branch light coming from the coupler 32.
The spectrum monitor unit 34 supplies the variable attenuators 36-1 through 36-3 with respective control signals, which control the levels of the respective optical signals on the basis of the results of the measurement in a feedback fashion. For example, the monitored level of the light of the wavelength λ1 is higher than the corresponding target level, the spectrum monitor unit 34 supplies the variable attenuator 36-1 with the control signal which controls the amount of attenuation thereof so that the monitored level reduces.
However, the WDM transmission device 10 shown in FIG. 2 has a disadvantage in that there is no way other than actual measurement of dispersion of the losses of the optical signals having the different wavelengths in the communication system. More particularly, the amounts of attenuation of the variable attenuators 20-1 through 20-3 are manually changed to adjust the differences among the losses of the optical signals. Thus, it takes a long time to perform initial installation, operation and maintenance of the system. Further, the WDM transmission device 10 does not have any means for coping with age deterioration and seasonal variation in performance.
The WDM transmission device 30 shown in FIG. 4 has the performance that depends on the required transmission distance. Thus, the device 30 is required to have the amplifier 14 of a type suitable for the situation in which the device 30 is used. The dynamic range of the amplifier 14 may vary. In the case where the variable attenuators 36-1 through 36-3 are feedback-controlled by the results of measurement by the spectrum monitor unit 34, the level of the input signal applied to the amplifier 14 may go beyond the dynamic range thereof.
Further, in order to maintain the OSNR at an appropriate level on the reception side, it is desirable that the optical signals of the different wavelengths have levels as high as possible. In this regard, the manual adjustment of the variable attenuators 20-1 through 20-3 of the WDM transmission device 10 shown in FIG. 2 is cumbersome and inefficient. In the WDM transmission device 30, the level of the input signal applied to the amplifier 14 may exceed the dynamic range thereof although the feedback control of the variable attenuators 36-1 through 36-3 enables the levels of the optical signals to be maintained as high as possible.